Relationships between Blue- and Red-based Vegetation Indices and Leaf Area and Yield of Alfalfa
2007
Hancock, Dennis W. | Dougherty, Charles T.
The need for site-specific yield assessments of alfalfa (Medicago sativa L.) has spurred interest in developing methods to remotely sense biomass at harvest. Relationships between reflectance-based vegetation indices (VIs) and yield and yield-components of alfalfa have not been fully characterized. The objectives of this study were to evaluate the relationships between blue- and red-reflectance based VIs and canopy variables such as leaf area index (LAI), mass shoot⁻¹, shoot length, and alfalfa yield . Canopy reflectance was obtained with two reflectance spectrometers 1 d before each of five harvests in 2005 within rainfed and subsurface drip-irrigated alfalfa. Blue- and red-based normalized difference vegetation indices (NDVIs) and wide dynamic range vegetation indices (WDRVIs) at three levels of a near-infrared (NIR) reflectance-scalar (‘α’ = 0.1, 0.05, and 0.01) were calculated and regressed on alfalfa canopy variables. A quadratic-plateau model was used to determine when VIs no longer detected yield increments. Both blue- and red-based NDVIs and WDRVIs exhibited significant (P < 0.0001) saturative responses to LAI, yield components, and dry matter (DM) yield. Decreasing α widened the estimable yield range (0–1.82 vs. 0–2.76 Mg ha⁻¹ and 0–2.60 vs. 0–3.74 Mg ha⁻¹, respectively) of both blue- and red-based WDRVIs. Significant (P < 0.0001) yield regression models within the effective range of the VIs (<Yieldₘₐₓ) were found within two harvests in 2005 and when data were pooled across all harvests. These results indicate that the use of a NIR reflectance-scalar can extend the range of herbage biomass (to 3.74 and 2.76 Mg ha⁻¹, respectively) within which blue- and red-based indices may be used to estimate alfalfa yield.
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